17071

Despite several decades of UV observations comma the rest-frame ionizing spectrum of QSOs remains uncertain comma primarily because of insufficient wavelength coverage to identify strong absorbers and measure spectral curvature between the EUV-FUV and NUV-Optical. The primary goal of this proposal is to define the continuum of QSOs comma for ionizing (Lyman continuum) wavelengths through the NUV and optical. This requires d_commafilling the NUV gapd_comma for intermediate-redshift AGN (z = 1.5-2.2) with existing COS-G140L (FUV) and optical (SDSS) spectra. To fit the continuum over a broad wavelength interval comma with access to windows of line-free continuum comma we need spectra (2000-3500A) that connect FUV and optical wavelengths. This region contains important absorption-line diagnostics comma including higher Lyman lines associated with LLS and pLLS absorbers. We request 26 orbits to obtain COS-G230L near-UV spectra of 12 AGN at redshifts z = 1.51-2.12 with existing far-UV (COS-G140L) and optical spectra (SDSS-6dF). This will extend the AGN composite spectrum down to 360-420A and investigate possible spectral hardening at wavelengths below 600A. We will also derive limits on EUV turn-overs and the He I continuum edge (504A) predicted in some models of accretion disks. Our spectra will also measure higher Lyman lines associated with strong (LLS-pLLS) absorbers and prominent QSO broad emission lines in the EUV that contaminate the underlying continuum.